Earth's gravity constantly pulls everything toward the ground, which is why getting to space is so difficult. To break free of that pull, a rocket must reach a speed of about 11.2 kilometers per second, known as escape velocity — roughly 40,000 km/h.
Rockets achieve this speed by burning huge amounts of fuel in a short time, producing thrust that pushes the rocket upward according to Newton's third law: for every action, there is an equal and opposite reaction. As burning gas shoots out of the engines, the rocket is pushed in the opposite direction.
Most rockets are built in stages. Each stage carries its own fuel and engines, and once a stage burns through its fuel, it detaches and falls away, making the rocket lighter so the remaining stages can accelerate more efficiently toward orbit.